1. Department of Obstetrics and Gynecology, Guangdong Women and Children Hospital, Guangzhou, Guangdong 510010, P.R. China
2. Department of Gynecology, Family Planning Research Institute of Guangdong Province, Guangzhou, Guangdong 510600, P.R. China
3. Department of Public Health, Guangdong Women and Children Hospital, Guangzhou, Guangdong 510010, P.R. China